Electric discharge lamp and manufacture thereof



V. VODICKA June 30, 1959 ELECTRIC DISCHARGE LAMP AND MANUFACTURE THEREOF Filed May 28, 1953 2 Sheets-Sheet 1 Inventor: -\/inc ent \/o dicKa,

-4 His Attorney June 30, 1959 v. VODICKA 2,892,956

ELECTRIC. DISCHARGE LAMP AND MANUFACTURE THEREOF Filed May 28, 1953 2 Sheets-Sheet 2 Inventor:

7 Vincent VodicKa, y/J /K His Att ney United tates Paten *ELECTRIC DISCHARGE LAMP MANUFACTURE THEREOF Vincent Vodicka, South Euclid, Ohio, assignor to General Electric Company; acorpora'tion of New York Application May 28,1953, SerialNo. 358,097

1 Claim. 01. ars -1'09.)

This invention relates toclectric discharge lamps and the manufacture thereof, particularly to fluorescent lamps'of the type in which a fluorescent coating is excited into luminescence.

In the application of fluorescent coatings to the interior surface of lamps of the character described, the mechanism of energy transfer from an arc to a fluorescent coating and the resulting absorption for ultraviolet light and the creation of visible light is of considerable importance. The materials comprising said coatings are generally described as phosphors.

The thickness of the phosphor layer is one of the vital variables. amount of the visible radiation is absorbed in the phosphor. If the layer is too thin, part of the ultraviolet radiation penetrates the layer, is absorbed by the glass envelope and is thus wasted. V

In view of these considerations, fluorescent lamps have in the pastbeen made by effecting a compromise in coating thickness. That is, the fluorescent-coating-was of such thickness that it would be impervious tomost of the ultraviolet radiation and at the'same time would not absorb an excessive amount of visible radiation. Observation indicates that the inside of the phosphor layer of a fluorescent lamp is brighter than the outer surface of said layer. Also it appears that not all of the ultraviolet radiation is utilized.

It is an object of my invention to provide new and improved electric discharge lamps.

It is another object of my invention to provide new and improved constructions for use in fluorescent lamps whereby greater efficiencies are available through more eflicient utilization of the radiation from the are dis charge.

It is a further object of the invention to reduce the loss of ultraviolet radiation in fluorescent lamps, thus increasing the initial brightness and efficiency.

It is a still further object of my invention to provide a new and improved fluorescent lamp inwhich the main tenance, i.e., the efliciency throughout life, is maintained at a higher value than that afforded by present commercially available lamps.

I have found that the above-mentioned difficulties may be surmounted by interposing a film of suitable ultraviolet-reflecting material between the glass surface of the envelope and the fluorescent coating, the reflecting film being less absorbing of visible radiation than the fluorescent coating. The use of such a reflecting film provides a means of transferring a substantial portion of the ultraviolet radiation to the outside phosphor surface'nearest the glass envelope where it is utilized. The reflecting film permits application of a thinner fluorescent coating than was previously feasible, thereby allowing more of the ultraviolet radiation to penetrate the fluorescent coating and be reflected back onto the outer phosphor surface rather than be absorbed by'the glass and consequently lost. Thus, the brightness of the outside phosphor surface is increased as well If the layer is too thick, an excessive.

ice

asthe over-all brightness of the lamp, provided the material used in the reflecting-film does not unduly absorb visible light.

According to one aspect of the invention, a thin film ofan ultraviolet-radiation-reflecting coating is applied to the interior surface ofa fluorescent bulb, and the fluorescent coating is, depositedover this film, thus giving a doubly coated lamp.

A substance having the desired properties of reflecting ultraviolet radiation and not unduly obstructing visible radiation is, for example, magnesium oxide. Upon comparison of lamps so coated with'fluorescent lamps as previously made, hereinafter referred to as a control lamp, it was found that the doubly coating lamps gave equal or better lumen output at zero and one hundred hours (of life) photometry. The thickness of the two coatings is such that: only between 50 and percent by weight of phosphor is-required as compared to the usual fluorescent coatings. Thus, a further result of this: invention is the provision of a fluorescent lamp in Which there is a substantial saving in fluorescent coating material concomitant with increased efliciency as opposed to the lowered efliciency previously encountered in the use of thinner phosphor layers.

In the manufacture of electric discharge devices such as fluorescent-lamps andthe like, one of the objects is to make a device which will be characterized by a high radiant flux or light output throughout its life. In the case of devices emitting visible light, the efliciency of production of luminous flux is usually expressed in terms of lumens per watt. The ideal device would be one which suffered no loss in radiant flux output during its life. However, it is. Well knownthat in fluorescent lamps in which phosphors are excited to luminescence by the short-wave radiation of a low pressure mercury vapor discharge, the. quantity of visible light emitted per unit of electrical energy used tends to fall with continued use of the lamp. In most cases a substantial loss is suffered early in life, generally between zero and one hundred hours of operation, and the loss increases more slowly thereafter as the-life expectancy of the lamp is approached. Small or large losses in the lumens per watt value are characterized as good or bad maintenance of the lamp.

It is also known that in some cases had maintenance is-associatedwith the character of the glass or the glassphosphorinterface. Rapid losses in maintenance are also often accompanied by a darkening of the fluorescent coating. These effects can be shown to occur with phosphorscontaining an excessive amount of alkali, or with glasses containing excessive amounts of alkali, unless steps are taken to clean the surface of the glass in a way which is likely to substantially reduce the amount of alkali thereon. It has been discovered that these effects can be minimized, and the maintenance fall reduced, by inserting between the phosphor and the glass a material which by its nature, while affording the desired buffer between phosphor and glass, does not materially absorb or alter the light radiation from the phosphor. The interposed material should be highly transparent to most of p the rays of visible light and should not be such as to react with the phosphor to the detriment of the latter in later stagesof lamp manufacture such as the removal of the phosphor binder. Magnesium oxide is a substance, possessing the above-mentioned properties, which is particular-ly suitable for improving the maintenance characteristics of electric discharge devices. This effect may be attributable, at least in part, to the ability of the magnesiato absorb gascsand moisture, thus acting as a getter within the bulb. However, on the other hand, I have found that other ultraviolet-reflecting materials, such as barium sulfate, barium fluoride, magnesium fluo- Patented June 30:, 1959:

3 ride, alumina and silica not only fail to improve the light output but actually resulted in decreased efliciency as compared to control lamps, particularly during the course of lamp life. g p

c For a better understanding of 'my invention, reference may be had to the following description taken in connection with the accompanying drawing, and the scope will be pointed out in the appended clairn'.

Fig. 1 is a cross-sectional view of a low pressure mercury vapor fluorescent lamp embodying my invention;

Fig. 2 is a somewhat diagrammatic plan View of the reflectometer useful in determining the proper thickness of the fluorescent coatings in electric lamps; and

Fig. 3 is a wiring diagram of the reflectometer so use Fig. 4 illustrates, in partly sectional elevation, another embodiment of my invention as applied to a high pressure mercury lamp of the color improved type in which the ultraviolet radiation-reflecting film is applied and adherent to a metallized reflector and the fluorescent coating is deposited over the reflecting film.

Another embodiment of my invention is shown in Fig. 5 which is an elevation, partly in section, of a high pressure mercury lamp in which an ultraviolet radiationreflecting film is applied directly to the inner surface of the enclosing envelope and a fluorescent coating is deposited over the reflecting film.

Referring to Fig. 1, I have there illustrated one embodiment of my invention as applied to electric discharge lamps, such as the low pressure mercury vapor fluorescent lamp disclosed and claimed in U.S. Patent No. 2,182,732, granted December 5, 1939, on an application of Meyer et al., and which is assigned to the assignee of the present application. A fluorescent lamp 1 is illustrated comprising a closed envelope 2 having therein oppositely disposed electrodes 3 which may be of the thermionic type, such as the coiled filament type, and which are preferably coated with an electron emissive material such as oxides of the alkaline earth metals and the like. The electrodes are connected and supported by suitable lead-in and supporting conductors arranged in stem presses 4 provided at each end of the lamp to external terminals 5. The arc discharge between the electrodes 3 constitutes a source of ultraviolet radiation.

As a means for increasing the efliciency of operation of the lamp in the production of visible radiation, I provide between the interior surface of the envelope 2 and a fluorescent coating 7, means selectively reflective of the ultraviolet radiation, such as a film 6 of material which is pervious to the visible radiation. For example, I provide between the envelope surface and the fluorescent coating a film, such as magnesium oxide, which reflects most of the ultraviolet radiation from the arc discharge which may be transmitted through and not utilized by the fluorescent coating. In this manner, most of the ultraviolet radiation transmitted through the fluorescent coating is returned to it for subsequent conversion to visible radiation. 7

Prior to scaling ofi the lamp, one or more stable starting gases, such as argon, neon or other noble, nonatornic gas at several, such as 1 to 5, millimeters of mercury pressure, is introduced into the envelope together with a quantity of an ionizable vapor which may be a small quantity of mercury 9 which exceeds the amount required during normal operation of the device. The electrodes 3 are designed to be heated by a suitable current during starting, and upon application of a suitable potential between said electrodes at positive column discharge occurs therebetween. The discharge vaporizes the mercury to furnish a mercury vapor atmosphere at a pressure on the order of 10 microns, with the result that ultraviolet radiation, particularly of the 2537 A wavelength, is generated. Said ultraviolet radiation will fall on the fluorescent coating 7 causing it to produce visible light.

Another embodiment of my invention is illustrated by Fig. 4 as applied to high pressure mercury vapor lamps of the color improved type, having a metallized reflector applied to the inner surface of the outer bulb. The device may be of the general type disclosed and claimed in U.S. Patent No. 2,491,868, granted December 20, 1949, upon an application of Ernest Martt, and which is assigned to the assignee of the present application.

Referring to Fig. 4 of the drawing, the lamp comprises an outer glass envelope 26 having a neck 27 and a reentrant stem 28. A screw type base 29 is mounted on the neck 27 and a pair of lead-in wires 30 and 31 are connected to the usual contacts on thebase 29 and extend through a press 32 of the stem 28 into the interior of the envelope 26 for conducting electricity to an arc tube 33 mounted within the envelope 26. The latter is provided with a reflecting coating 34, such as vaporized aluminum or silver, covering its inner surface from its neck 27 to its part of greatest diameter to control the light emitted by the discharge in the arc tube.

The are tube 33 is of a well-known commercial type and comprises a tubular envelope of high softening point material, such as quartz, having two main discharge supporting electrodes 35 and 36 mounted at its ends and an auxiliary electrode 37 mounted adjacent the main electrode 35 and connected to the other electrode 36 through a resistance 38 for starting purposes. The electrodes are sealed into the envelope by the usual sealing glass. The tube contains a starting gas at a few millimeters of mercury pressure and mercury in such an amount that it is completely vaporized at a temperature slightly below that at which the tube is designed to operate. During operation the mercury vapor pressure is suflicient to constrict the discharge so that it appears as a luminous chord or thread of high brightness along the axis of the tube. The outer envelope 26 protects the arc tube 33 from drafts or the like during operation and encloses the inner conducting parts of the lamp. It is usually filled with a gas such as nitrogen. The arc tube 33 is firmly supported by a suitable supporting means in predetermined relation to the reflecting surface 34 on the envelope 26 to offer the minimum obstruction to the light emitted by the discharge in the tube 33. A film 39 consisting of ultraviolet radiation-reflecting material is interposed between the metallized reflector 34 and a fluorescent coating 40 thereon to act as a bufier between the fluorescent coating and the metallized reflector and also to return to the fluorescent coating any ultraviolet radiation transmitted through it for subsequent conversion to visible radiation. I have also found that light from mercury vapor lamps of the type embodying my invention has a substantial component of red rays and hence approaches more nearly white light than does the light from present mercury arc lamps. It will be noted also that in this type lamp a very heavy ultraviolet-reflecting film may be deposited over the metallized reflector since noconsideration need be given to the light transmission properties in that portion of the bulb. A film of reflectance of 60-70% will increase the component of red rays as well as the total light output.

In Fig. 5 of the drawing I have illustrated another embodiment of my invention as applied to a high pressure mercury lamp of the same general type as shown in Fig. 4 comprising an outer envelope 41 which may be used in place of the envelope 26 of Fig. 4, a quartz arc tube 42 suitably connected and supported containing discharge electrodes 43 and 44 and a starting gas and mercury as in tube 33 of Fig. 4, a discrete film 45 of ultravioletreflecting material deposited on the inner surface of the enclosing envelope 41 and a fluorescent coating 46 applied over the reflecting film. In this manner the output of the lamp is increased by reflecting to the fluorescent coating any ultraviolet radiation which may penetrate it.

In accordance with one aspect of the invention, a fluorescent lamp bulb may bepre-coated with magnesium oxide by any of the following suitable methods: Applied I violet-reflecting property to a substantial extent. ample, as little as 2% by weight titanium dioxide mixed to the finished glass tube as magnesium oxide" powder; as smoke from burning magnesium; electrostatically as disclosed in U.S. Patent No. 2,538,562, Gustin et a1.; centrifugally as a powder suspended in a vehicle; or'flushed as ,a powder suspended in a vehicle. The magnesium oxide may also be applied as a solution in the form of a spray or mist of a solution in water or an organic solvent of a magnesium salt, preferably acetate or other salt of an Organic acid, to the interior of a hot glass tube with subsequent decomposition of the salt to magnesium oxide in a thin transparent or translucent film on the tube wall or as a spray or flush coating of a solution of a magnesium salt, as previously described, on the interior wall of a tube at room temperature, followed by drying and heating to a temperature sufficient to decompose the salt to a film of magnesium oxide. The same method may be used where the magnesium exists in solution as an organic complex, not necessarily a salt. The coating may also be applied to the glass as a powder during manufacture of the tubing by forcing smoke from burning magnesium into the soft tubing during the forming stage or by dispersing the powder in air by any means other than burning the metal. It may also be applied as a solution by atomizing a salt of magnesium to form a fine mist and forcing the mist into the interior of the tube during forming.

The magnesium oxide film is preferably applied by the flush method in which magnesium oxide powder is mixed with any suitable organic binder and the resultant slurry is thinned by the addition of a suitable thinner therefor to give the desired thickness of reflector film when the slurry is flushed down the interior surface of the tube. I prefer to use about 0.1 to about 0.3 gram of magnesium oxide per 40 watt bulb which is nominally 48 inches in 'length and 1 /2 inches in diameter. The film thickness is measured in terms of its reflectance value R, and the method and apparatus by which this value is determined willbe discussed in detail at a later point. The reflector film thickness that gives the desired result, that is, maximum visible radiation output and maintenance, is the range of R from -30%, based on the reflectance to visible radiation, which corresponds to 0.07 to 0.24 milligram per square centimeter of bulb surface. The fluorescent coating is then deposited over the reflector film in such thickness as to give a total reflectance in the approximate range of R=58 to R=72 percent (under visible radiation) which corresponds to 1.25 to 5.05 milligrams of fluorescent material per square centimeter of bulb surface. The reflectance values of the reflector film and the fluorescent coating are not additive arithmetically. That is, a reflector film of reflectance of 30% covered by a fluorescent coating of reflectance of 60% does not give a total reflectance of 90% but rather a value somewhat higher than the greater of the two coating values. In this instance the total reflectance of the two films would be about 69% The fluorescent coating may be deposited by any suitable method. For example, it may be applied by the flush method wherein the luminescent powder is mixed The tube I have found that the admixing of an ultravioletabsorbing or non-reflecting substance with the ultraviolet radiation-reflecting film causes the film to lose its ultra- For exwith the magnesium oxide of the reflecting film reduces the gain in lumen output over control lamps at 100 .hours of operation by about 62%. At 4% by weight the amount of ultraviolet-absorbing or non-reflecting material in the reflecting film was increased to 8% by weight, all beneficial effect of the reflecting film was destroyed and lamps so coated gave slightly less lumen output at hours of operation than control lamps. Increasing the amount of titanium dioxide to 50% by weight reduces the lumen output of the double coated lamp to the point where it is but 92% of the output of a control lamp.

Mixing the magnesium oxide with the fluorescent material and applying the mixture as a composite film does not produce the dual improvement of my invention. While it is true that magnesia admixed with the fluorescent material and applied to the glass surface of the bulb will effect some improvement in the maintenance characteristics of the lamp, a coating of this mixture does not produce the desired increase in initial lumen output and efliciency of the lamp. Thus, for the above reasons, I prefer to use an unadulterated film of an ultravioletreflecting substance between the inner glass surface of the bulb and the fluorescent coating thereon.

One method of depositing magnesium oxide, and determining optimum thicknesses thereof, will now be considered in detail. Magnesium oxide of high purity and fine particle size, such as that prepared according to copending U.S. application Serial No. 335,603, Froelich, filed February 6, 1953, now Patent 2,765,212, and assigned to the assignee of the present invention, is mixed with nitrocellulose binder in a ratio of 1 gram of magnesium oxide to 3 cubic centimeters of nitrocellulose solution. The mixture is ball milled for about two hours in a one gallon mill, after which additional binder is added to bring the ratio of magnesium oxide to hinder to 1 gram to 5 or 6 cubic centimeters. The resulting slurry is thinned with butyl acetate to give the desired reflectance when flushed over the inner surface of the glass envelope. The binder used for magnesium oxide suspension is a solution of 0.4 to 0.5 percent by weight nitrocellulose (dynamite grade) in butyl acetate. It will be noted that the binder used to suspend the fluorescent materials for flush coating is of a different character (although usual for this purpose) comprising a solution of one pound of nitrocellulose (dynamite grade) in eight gallons of petroleum distillate (naphtha) and 12 gallons of butyl acetate the distinction being that naphtha is present in the binder for the fluorescent coating. Naphtha is not as good a solvent as butyl acetate, thus the probability of dissolving some of the pre-coated magnesium oxide film when the fluorescent material is flushed over it is reduced. The magnesium oxide film that gives the desired result (that is, maximum improvement in light output and maintenance) is of a reflectance of 10-30% and is about 0.07 to 0.24 milligram per square centimeter of bulb surface. Optimum results are obtained with a magnesia film of about 20-30% reflectance which is about 0.14 to 0.24 milligram per square centimeter of bulb surface. After the magnesium oxide film has been completely dried in air for about 10 to 15 minutes, the bulb is inverted to compensate for the differential in coating thickness from one end of the bulb to the other and the fluorescent coating is then applied over the magnesium oxide film in the usual manner. The phosphor deposit that brings the total reflectance to within the range of 64-67% (about 1.50 to 3.30 milligrams of phosphor per square centimeter of bulb surface) gives the highest lumen output. The material used in the fluorescent coating of this particular example was calcium h-alophosphate, one of the alkaline earth halophosphate phosphors disclosed and claimed in U.S. Patent 2,488,- 733, McKeag et al., granted November 22, 1949, and assigned to the assignee of the present application, but it will be understood thatany suitable fluorescent material may be employed. After the coatings have been applied, the bulb should be heated at a temperature and for a time sufficient to thoroughly drive off the binder and gases (about 400-600 C.), followed as quickly as possible by exhausting and sealing of the lamp in a continuous process.

The fluorescent lamp as. presently made is coated only with a film of fluorescent material of a reflectance of about 60%. In the double coated lamp of my invention the total reflectance of the two coatings may be as low as 58% without undue loss of lumen output as compared to the control lamps. That is, although the double coated lamp of total reflectance of 58% will have the same or only slightly better initial lumen output than:the control lamp, it will exhibit considerable improvement in maintenance over the control lamp. Such a lamp at the same time has an additional advantage of requiring only one-half to two-thirds the amount of phosphor by weight as compared to standard coatings. The saving in phosphor material for highest lumen output (that is, .a magnesium oxide film of reflectance of 20-30% and a phosphor layer sufliciently thick to bring the total reflectance to 64-67%) is aboutt 18-22% by weight over control lamps. Where the total reflectance of the two films exceeds 72%, no practical improvement is present inasmuch as one or both of the films have become so thick as to absorb an excessive amount of the visible radiation thus reducing the lumen output below that of control lamps.

The magnesium oxide pre-coated lamps prepared in accordance with the invention give higher initial lumen output and lower 100-hour drop than control lamps. The improvement is about 50 lumens at zero hours and 120 lumens at 100 hours of operation in lamps of the 40 watt size. The efliciency in lumens per watt for magnesium oxide pre-coated lamps at 100 hour photometry was almost equal to zero hours efliciency of control lamps. In view of these results, an improvement of at least two lumens per watt over control lamps can be expected at all times.

Inasmuch as the coating thicknesses are discussed in terms or reflectance, the construction and operation of the device by which such values are determined will now be-set forth in detail.

The instrument used is a General Electric Type X-93 reflectometer which is intended for use in checking the thickness of the phosphor, or other, coating on fluorescent lamps. Since the amount of light that will .be reflected by a coated bulb depends upon the thickness of the coating, it is possible to get relative indications of coating thickness by measuring the amount of light reflected by the coated bulb under certain standardized conditions. The Type X-93 reflectometer is designed to measure, by comparison with a standard, the 30-degree reflectance of a coated bulb in a horizontal plane including the axis of the test bulb and the light source beam when the incident light is normal to the bulb surface. The standard of comparison is a special coated bulb of known reflectance.

The reflectometer (Figs. 2 and 3) consists essentially of a source of controlled illumination anda means of measuring the reflected light. Except for the microammeter 22, variac 21 and voltage stabilizer 20, all of the parts are mounted compactly on a framework of wood, the central part of which forms a light-tight cell box 18. Fig. 2 shows the location of the condensing lens 12. photoelectric cell 17 and stand-by lamp 13 in the cell box. The light source lamp and rheostat 2.3 are mounted in a separate compartment 11 at the rear. Holes for ventilation are provided in the top, bottom and back of this lamp housing 11. This arrangement keeps the heat away from the photocell 17 and thus allows it to operate at a more uniform temperature. All wood and metal surfaces, both internal and external, are painted flat black. In addition, the inside of the wall to which the aperture plate is attached is completely lined with n. der tominimize the effect of variable room light on the meter reading. ,Bulbs to be tested, of outside diameter of limb through 2% inches, are supported on a sliding brackets 14 on either side of the housing. Bulbs of a smaller diameter may be held by a pair of l -blocks. (not shown) mounted on a guide with a spring arranged to hold the assembly firmly against the bulb.

The reflectometer Operates from a standard 115 volt source of alternating current, and it is essential that this voltage supply bewell regulated. Since most circuits do not usually have good enough regulation for this type of work, a separate voltage regulator 20 must be used with the instrument. The regulator used, a General Electric 50 volt-ampere automatic voltage stabilizer (Catalog No. 67G750), will hold its rated output voltage within /2% even though the line voltage varies from to 130 volts. This regulator, because of the capacitive circuit it employs, draws a heavier current (and hence runs hotter) with no secondary load than with full load. For this reason a switch .19 is provided on the primary side so that the regulator can be turned off when the equipment is not in use. The lamp 10 used as a source of illumination is a 50'watt, volt, T8 bulb (1 inch diameter) projection lamp having a coiled-coil tungsten filament, said filament operating at about 4500 F. A condensing lens 12 in a sliding holder is provided to focus the light on the aperture. The light should not be focused sharply as an image of the light source filament is to be avoided. The optimum focus is one which combines as strong a light as possible with as uniforrna light as possible falling over the aperture. The intensity of the light beam can be adjusted by means of the variac 21 or the 50-ohm rheostat 23 in series with the lamp filament. (Where the reflectance of visible light is hereinafter referred to in the appended claims, it is contemplated that such values will be determined by using alight source 10 as described above.)

When the reflectometer is used intermittently, it is desirable to have a more or less uniform light falling on the photoelectric cell. This avoids any inaccuracies that might arise from making a reading with the reflectometer before the cell has reached a stable condition. When the instrument is not in use, voltage can be applied to a 6WS6 stand-by lamp 13 in the cell box by means of the switch 25. By adjustment with the variac 21 and rheostat 23, the illumination falling on the cell can be maintained the same as when the instrument is being used. If the stand-by lamp 13 is properly located in the cell box, as shown approximately in Fig. 2, the adjustment used for the light source lamp 10 should be close enough for the stand-by lamp.

The amount of bulb surface exposed to the light beam for measurement is controlled by an aperture plate at the front of the light-tight box 18. Apertures of different sizes are used for different bulb diameters. In order to provide the best integration of point-to-point variations in coating thickness, it is desirable to use as large an aperture as possible for a given bulb diameter. The maximum aperture that it is practical to use, however, is limited in the vertical direction by the curvature of the bulb, and in the horizontal direction by the dimensions of the photocell. Suitable aperture sizes for the various bulb diameters: are as follows:

Aperature Height, inches XXX The photocell 17, a General Electric barrier layer cell, is mounted on a right-angle bracket in the position shown in Fig. 2. The long dimension of the cell face is vertical. The cell-to-aperture distance has been chosen to give the desired meter deflection within the range of light intensity that can be provided. The cell may be reset at any other point along the 30-degree angle if changes in aperture size, light intensity or cell sensitivity should require it. To avoid introducing large errors caused by specular reflection from the glass surface of the test bulb, the cell must be placed in the same horizontal plane as the light source beam and the test bulb axis.

The photocell current is read on a direct current microammeter 22 having a range of O to 100 microamperes. By using a meter of this range the reflectometer can be made direct-reading for reflectance values in terms of percent of total incident light reflected by the bulb coating. The meter normally used with this equipment, a General Electric Model DP-9 microammeter, has characteristics such that a damping resistor is required to prevent excessive swinging of the meter needle before it comes to rest. A 1000 ohm wire-wound resistor 24 provides this damping action.

Since a small amount of light will be reflected into the photocell from the walls of the housing and the stand-by lamp, the meter used must have a large enough zero adjustment to permit the needle to be set on zero under zero" conditions (that is, with the light source turned on and no bulb in front of the aperture). If the meter is not adjusted in the above manner, an error is introduced which is proportional to the amount of stray light and to the difierence between the reflective value of the standard used and the test bulb. The error causes test lamps to read closed to the standard than is the actual case. For example, using a reflectometer with a stray light reading of 10% and and a standard reading 60% without zero adjustment causes a test bulb with a reflectance of 54% to read 55%. Similarly, a test bulb having a reflectance of 66% will read 65%. This error can be minimized by decreasing the internal reflection of the instrument and by using a standard whose reflectance is close to that of the test bulb.

The coating thickness standards used with this reflectometer are dummy lamps which are similar to regular lamps in all respects except that they contain no mercury and no electrodes. Each standard may be used to check any coated bulb of the same diameter, regardless of its length. The standards have reflectance values marked on them which have been determined by comparison with a set of master standards. Each standard is also marked to show which area of the bulb Was used in the calibration. This same area should face the aperture whenever the standard is used.

Although the reflectometer is not particularly sensitive to stray light, it should not be used near a window where the level of illumination might change quickly. The light shield in front of the lamp prevents any stray light from striking the photocell directly, and with only a moderate and reasonably constant room illumination this is suflicient protection.

When the instrument is to be used intermittently it should be left with switch 25 in the stand-by position between uses. When first turned on, a warm-up period of about 15 monutes using the stand-by lamp will aid in stabilizing the cell.

The reflectance value of a coated lamp may be determined in the following manner:

(a) Select a standard bulb of the same diameter as the bulb to be measured.

(b) See that the aperture plate is the correct size for the diameter of the bulb to be measured.

(0) Insert the standard bulb, taking care to have the marked area in front of the aperture.

(d) Adjust the rheostat 24 until the meter 22 gives exactly the reading marked on the standard bulb.

e) Leave the light source on and at the same intensity remove the standard bulb and adjust the zero setting of the meter 22 so that it reads exactly zero.

(f) Replace the standard bulb and readjust, if necessary, to bring the meter reading back to the known reflectance value.

(g) Remove the standard bulb and substitute the bulb to be checked, placing the center of the bulb opposite the aperture.

(h) Rotate the bulb through one full revolution, observing the deflection of the micrometer as the bulb is turned. The approximate average reading (that is, the value at which the meter needle is most nearly constant) should be recorded as the coating thickness of the bulb under test.

(i) If a series of readings are being taken, the setting should be rechecked with a standard bulb at frequent intervals to compensate for cell fatigue. To prevent a possible error of more than 1%, the calibration should be rechecked at intervals not greater than three minutes for the first six minutes of continuous operation, and at inntervals of 10 minutes thereafter.

Although a preferred embodiment of the invention has been disclosed, it is recognized that variations and changes may be made therein within the spirit and scope of the invention as defined by the appended claim. It is understood particularly that the thickness and composition of the coatings can be varied, independently and in relation to each other, within fairly wide limits to obtain the desired results.

What I claim as new and desire to secure by Letters Patent of the United States is:

In an electric lamp, the combination comprising an outer envelope of light-transmissive material, a source of ultraviolet radiation comprising an inner container of ultraviolet radiation-transmitting material having therein a pair of electrodes and an ionizable medium, a metallized reflector on the inside surface of the outer envelope, a film of light-pervious, ultraviolet-reflecting magnesium oxide on said reflector and a coating of fluorescent ma terial on said film for converting ultraviolet radiation into light, said film having a reflectance in the range of 60 to whereby to reflect to said coating a substantial portion of ultraviolet radiation transmitted therethrough in order to increase the efiiciency of lamp operation.

References Cited in the file of this patent UNITED STATES PATENTS 2,223,425 McKeag Dec. 3, 1940 2,235,802 Lemaigre-Voreaux Mar. 18, 1941 2,476,681 Overbeek July 19, 1949 2,494,883 Kroger Jan. 17, 1950 FOREIGN PATENTS 603,326 Great Britain June 14, 1948 

